Silk-screen printing, more accurately designated "screen printing" has long been used to produce high quality illustrations although its method does not readily lend itself to high volume production. In the last decade, however, screen printing has had a resurgence in use for applications where long runs of high quality prints are desired, this resurgence being due to the use in the process of more complex, automatic equipment. Essentially, screen printing is a form of stencil printing using a design fashioned from paper, or other material, and attached to a woven sheet of fabric (silk, organdie, nylon or polyester fibers, copper, stainless steel strands, etc.) which has been tightly stretched across a frame. Ink is then forced through the open meshes of the fabric sheet by means of a rubber blade or squeege. The size of the opening in the mesh of the woven fabric, if formed of silk, might vary from 6xx (coarse) to 18xx (very fine). The mesh can also be achieved by etching a thin metal plate but more commonly is formed by a woven fabric. Pervading all screen printing applications, particularly where a flexible fabric is used, whether using high volume automated equipment or a simple rectangular printing frame, is the requirement that the woven fabric be fastened evenly and drum-tight to the frame. Since the woven fabric can easily be stretched, distorting and stretching the openings in the mesh, the even, drum-tight application of the woven fabric to the frame is not an easy task and requires skill and experience whether the mounting of the fabric is accomplished by mechanical apparatus or by simple tacking procedures. Further, different materials have differing optimum stretch characteristics for screen printing and the optimum degree of stretch is, customarily, recommended by the manufacturer or weaver of the material. Stretching of the material to the degree recommended by the manufacturer while maintaining the mesh square with the frame, both vertically and horizontally, is very difficult when the stretching of the fabric is done by hand. Since the mesh is very fine, use of a magnifying glass is often necessary to detect distortion of the mesh and this compounds the difficulty. Measuring the distance between the colored bands in the fabric of the present invention provides a convenient means for determining the proper amount of stretch imparted to the fabric. Any variation in the amount of pull exerted by the operator as he fastens the fabric to the frame causes the mesh to be wavy in appearance. While mechanical stretching apparatus gives improved results, even with such apparatus difficulties are encountered at the corners of the frame. The presence of the spaced, colored bands in the fabric of the present invention eases this difficulty.
Most materials used to form the mesh will shrink or stretch as ambient humidity and temperature vary. When mounted screens, with stencil attached, are stored or set aside even for a short time, if the mesh is not square with the frame, humidity or temperature change induced stretching or shrinking occurs on a bias causing mesh distortion. Detection and remedying this distortion is made easier by the presence of the horizontally and vertically spaced colored bands in the fabric.
The concept of the present invention envisages providing a woven fabric particularly adapted for silk screen printing in that it has groups of one or more adjacent vertical and horizontal threads provided with a color contrasting with the overall cover of the fabric. As the fabric is mounted on the frame, the rectilinearity of these contrastingly colored "stripes" in the fabric provide a convenient means for visually monitoring the condition of the fabric and preventing distortion and stretching of it as fastening or mounting proceeds.